<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    div{
        width: 500px;
        height: 150px;
        background-color: red;
        display: flex;
        /* justify-content: space-around;  */
        justify-content: stretch; /*水平*/
        align-items: center;      /*垂直*/
    }
    div>span{
        background-color: green;
        width: 100px;
    }
</style>
<body>
    <div>
        <span>1</span>
        <span>2</span>
        <span>3</span>
    </div>
</body>
</html>